1. Field of the Invention
This invention is directed to the vapor phase formation of compound semiconducting films in general and particularly to the formation of indium phosphide and gallium phosphide films on single crystal substrates.
2. Description of the Prior Art
Compound semiconductor films in general have been used extensively in various electronic devices as, for example, the fabrication of laser diodes, microwave oscillators and photoelectric detectors. In microwave applications, thin film semiconductors are required for high frequency response characteristics.
Semiconductor quality InP epitaxial thin films have been deposited via Chemical Vapor Deposition (CVD) (R. C. Clarke, B. D. Joyce and W. H. E. Wilgoss, Solid State Communications, Vol. 8, p. 1125, 1970) and Liquid Phase Epitaxy (LPE) (J. L. Shay, K. J. Bachmann and E. Buehler, Appl. Phys. Lett., Vol. 24, p. 192, 1974). However, these techniques require temperatures above 500.degree. C. For some applications, lower substrate temperatures are required. For example, substrate decomposition limits the substrate temperature for certain heterojunction photoelectric devices. Specifically, a terrestrial solar cell incorporating InP deposited on CdS would have a potentially high energy conversion efficiency (Sigurd Wagner, J. L. Shay, K. T. Bachmann, E. Buehler, Appl. Phys. Lett., Vol. 26, p. 229, 1975), but fabrication of such a device would have to be accomplished below approximately 400.degree. C to avoid CdS decomposition. Another application involving lower substrate temperatures would be abrupt junction microwave devices such as InP Gunn Effect devices. In this instance, lower temperatures would alleviate interdiffusion effects.
Molecular beam techniques have been used successfully by various workers (A. Y. Cho, J. of Vacuum Science and Technology, Vol. 8, p. 531, 1971; D. L. Smith and V. Y. Pickhardt, J. of Appl. Physics, Vol. 46, p. 2366, 1975) for the deposition of semiconductor epitaxial films at low temperatures. However, problems arise with this technique in producing the appropriate phosphorus vapor species (C. T. Foxon, B. A. Joyce, R. F. C. Farrow and R. M, Griffiths, J. Phys. D., Vol. 7, p. 2422, 1974) for InP deposition. Additionally, this technique severely limits the uniform deposition area and therefore is not scalable. This arises because two sources are used. These sources are at two different temperatures and, therefore, must be thermally isolated from each other. This isolation requirement leads to the limited uniformity in the deposited films (see U.S. Pat. No. 3,615,931 by J. R. Arthur et al).
Reactive evaporation techniques have been used to deposit polycrystalline semiconductor compounds on single crystal substrates. (F. J. Morris et al, J. Vac Sci. Technol., Vol. II, No. 2, page 506, March 1974.) However, films deposited via this technique were neither single crystal films nor of high purity and therefore were not of semiconductor quality. The techniques disclosed by Morris et al. are not readily scalable and fail to control sources of impurities during the deposition process.
There are no prior art processes known to me which facilitate the simultaneous production of high purity semiconductor quality films at low processing temperatures by a scalable method.